Are you planning to install solar panels on your roof? Understanding what a kilowatt-peak is will help you! This unit of measurement tells you how much power your panel can deliver under optimal conditions.

In other words, the higher a panel’s kWp, the better it performs. Installers also talk about ‘nominal power’.

kWp = max. electrical power under standard conditions

Of course, it is impossible to predict in advance the exact amount of electricity that a photovoltaic system can produce! Indeed, there are many elements involved:

• the location
• panel orientation
• the direction of shade
• the weather
• the presence of clouds
• the temperature on the roof
• …

The watt-peak (Wp) is therefore an indication based on a standard. It corresponds to the maximum electrical power that can be supplied by a photovoltaic panel under standard temperature and sunlight conditions. 1 kWp = 1,000 Wp

What are these standard conditions?

• solar radiation of 1,000 watts/m²
• an ambient temperature of 25°C (photovoltaic panels produce less output beyond this limit)
• a clear sky, around noon for example

How can the concept of Wp be applied in practice?

To choose your panels

The Wp listed by manufacturers makes it possible to compare different photovoltaic panels. For the same surface area, the higher the Wp, the better the panel performs.

To calculate the required panel area

Do you want to achieve a certain yield with your photovoltaic system? The Wp of each panel will allow you to calculate the surface area needed to reach it.

1 kWp corresponds theoretically to 1,000 kWh per year.

NB: do not forget that the Wp remains a theoretical value that corresponds to optimal solar radiation. Therefore, you must take into account the specific conditions under which your panels are installed.

Thus, a 1 kWp set of panels will produce an average of 900 kWh per year under optimal conditions (south, 35° angle), on the roof of a house in Brussels. On the roof of your second home in southern Europe, however, it will produce 1,250 kWh/year!

To calculate the maximum possible yield of your roof

Do you have a small roof? Are you wondering if it’s worth installing panels? The number of kWp will help you evaluate the yield you could achieve if you cover it with photovoltaic panels.